THE REPTILES AND THE BIRDS 121 



itive mammals— the duckbill, platypus, and the spiny 

 anteater, echidna— the egg is covered with a thickened 

 shell which may or may not be calcified, but it always 

 remains porous enough to permit the passage of oxygen 

 and carbon dioxide. This shell is, however, relatively im- 

 permeable to water, thus preserving the water supply of 

 the amnion. Where the Amphibia had relied on external 

 fertilization of the egg, this cleidoic (kleistos = closed) 

 egg had to be fertilized in the female before it was cov- 

 ered with its shell, an operation that was effected primi- 

 tively by the intromission of sperm into the cloaca with- 

 out the aid of special sexual organs. 



Because of the basic identity of pattern in the amni- 

 otic egg— which is one of the most impressive features in 

 vertebrate history— the reptiles, the birds (which are of 

 reptilian origin), and the mammals are frequently if in- 

 formally called the amniotes.' It is clear that the am- 

 niotic egg (the oldest known fossilized specimen comes 

 from the Lower Permian) must have been invented be- 

 fore the ancestral lines, which led, respectively, to the 

 reptiles and the mammals, had diverged— perhaps by a 

 form akin to Seymouria (see Figure 8) in the late Penn- 

 sylvanian when the Appalachian revolution was bringing 

 aridity upon the land— and it is equally clear that the 

 first 'amniote' was no more a reptile than it was a mam- 

 mal. The great physiological differences between the 

 reptiles and mammals, differences which separate them 

 as markedly from each other as from the Amphibia, war- 

 rant the not entirely frivolous suggestion that a new class 

 (or superclass) be recognized, intermediate between the 

 reptiles and mammals on the one hand and their am- 

 phibian root on the other, and perhaps to be called the 

 Amniota (Pennsylvanian to Permian; famihes, genera, 

 and species incertus), of which no rehct survives. In the 

 absence of the paleontologist's blessing, we shall sim- 

 ply refer to these intermediate forms as the 'nascent' 

 amniotes. 



